failure sending mail using system.net.mail(SMTP)

string ActivationUrl;

            try
            {

                MailMessage message = new MailMessage();

                message.From = new MailAddress("XXXX@gmail.com");

                message.To.Add(txtemail.Text);

                message.Subject = "Verification Email";

                ActivationUrl = Server.HtmlEncode("http://localhost:9525/Important_Testing/Verification.aspx?USER_ID=" + GetUserID(txtemail.Text));

                message.IsBodyHtml = true;

                message.Body = "<a href="" + ActivationUrl + "">Click Here to verify your acount</a>";

                SmtpClient smtp = new SmtpClient();

                smtp.Credentials = new System.Net.NetworkCredential("XXXX@gmail.com", "XXXX");

                smtp.Host = "smtp.gmail.com";

                smtp.Port = 587;

                smtp.EnableSsl = true;

                smtp.Send(message);

            }

            catch (Exception)
            {
                throw;
            }